Porous prilled ammonium nitrate

Abstract: A porous prilled product, particularly ammonium nitrate, and its method of preparation, which includes encapsulated microspheres, particularly polymer microballoons, which serve to reduce the density of the product. The invention also extends to an explosives composition particularly ANFO and heavy ANFO, including such ammonium nitrate of reduced density.

Description: THIS invention relates to a porous prilled product, particularly porous prilled ammonium nitrate. In this specification the term 'porous prilled' is used to refer to a particulate product which, in the case of ammonium nitrate, contains less than 0.5%. preferably less than 0, 2%, water on a mass per mass basis. Porous prilled ammonium nitrate is commonly employed as one of the components of explosive compositions used in the mining industry. Thus. for example, porous prilled ammonium nitrate is mixed with fuel oil to form the explosive composition known as ANFO, and with an emulsion to form heavy ANFO, with or without the prior addition of fuel. For reasons of cost efficiency, and/or in order to control the total explosive power of a particular charge; a low density ammonium nitrate prill is often preferred and various methods have been devised for producing such a product. In general such methods relate to the prilling of less concentrated solutions of ammonium nitrate. Such a method is for example this described in Australian Patent 621,994. In this patent, ammonium nitrate and low density particulate bulking material are blended together while said ammonium nitrate is in a molten state. The ammonium nitrate containing low density particulate material is subsequently formed as particles. This patent describes a two steps process for obtaining prilled product. Apart from the fact that the aforesaid known methods have limits of practicality, they also place a heavy burden on available drying equipment. Also, the resulting prills, although having the desired higher porosity and hence lower density, are usually of relatively low mechanical strength, often to the extent where they are useless in practical application. It is accordingly an object of this invention to provide a porous prilled product particularly ammonium nitrate, of reduced density and its method of preparation, with which the aforesaid problems can be overcome or at least minimised. A further object is to provide an explosive composition including such a pnlled ammonium nitrate of reduced density. According to the invention a porous prilled ammonium nitrate product is provided which includes hollow polymer microspheres being polymer balloons which had expanded during the prilling process to a size between 2 and 150 micro meter, incorporated in the crystalline structure. Applicant has found that the presence of such microspheres reduces the density of the product substantially. Preferably the microspheres have the following physical properties in the final product: Density range: 0.015-0.39 gram per cm 3 Temperature stability: Stable at process temperatures of 130-170°C for a sufficient time to effect prilling during the prilling process Breaking strength: Able to withstand at least 100 kg/cm 2 force or, able to regain its shape after impact deformation. Further according to the invention the porous prilled product also includes lime stone. In a prefered form of the invention the porous prilled product may include from 70.0% to 99.9% mass per mass of amonium nitrate containing encapsulated microspheres as disclosed above and from 30.0% to 0,1 % mass per mass limestone. Still further according to the invention the porosity of the product can further be enchanced by the inclusion of gas in the product during the prilling process. Applicant has found that the passages created in such a prilled product by the inclusion of gas can improve the porosity of such product without unduly effecting its mechanical strength. Further according to the invention the gas is developed in situ in the product via a suitable chemical reaction. Still further according to the invention the gas comprises carbon dioxide which is formed through the decomposition of a suitable carbonate in acid medium. The carbonate may comprise any suitable water soluble inorganic salt of carbonic acid, such as, for example, potassium and/or sodium carbonate or, alternatively, it may comprise a less soluble salt. Since the porous prilled product comprises ammonium nitrate, it invariably contains some acidity which reacts with the added carbonate to yield the required carbon dioxide. Preferably the carbonate comprises potassium carbonate which is present in a concentration of between 0,01 to 1,00% mass/mass. Applicant has found that the potassium nitrate which is formed in the reaction between the added potassium carbonate and nitric acid present in ammonium nitrate acts as a crystal habit modifier for the ammonium nitrate, thus imparting increased mechanical strength to the ammonium nitrate and increasing the transition temperature (32 degrees C) of the crystal transition between the II crystal form and the III crystal form. Still further according to the invention the prilled product may include colloidall silica which serves to harden the surface of the product. Further according to the invention the colloidal silica is provided by adding to the unprilled product silicilic acid and/or poly silicilic acid and/or waterglass in a sufficient concentration, preferably between 0.1 and 10% mass/mass, to ensure an acceptable friability and breaking strength. The invention also includes within its scope an explosive composition, particularly ANFO and/or heavy ANFO; including such a prilled amonium nitrate product of which the density had been reduced due to the presence of encapsulated microspheres. Applicant has found that, apart from obtaining a lowered density in the resulting prill, with or without the addition of gas producing carbonate and with or without the addition of colloidal silica, the lower density is also retained after pneumatic loading of an ANFO-type explosive formed from the porous prill and fuel oil in the 94:6 ratio normally used for such an explosive. One embodiment of a method to produce a product according to the invention will now be described by way of example with reference to the enclosed figure 1 which is a diagrammatic longitudinal section through an apparatus suitable for carrying out the method. In this arrangement a conventional type of conically shaped (shower head) prilling nozzle 10 is employed for the prilling of ammonium nitrate. Nozzle 10 has a tubular inlet 11 through which the ammonium nitrate solution to be prilled can be fed onto two spaced apart transversely extending diffuser plates 12 and 13 and to pass from there via a 100 micro metre screen 14 and nozzle plate 15 as ammonium nitrate droplets 16. Nozzle 10 is also provided with an elongated discharge tube 17 which runs down the side of nozzle 10 and which is bent over its length as indicated. Tube 17, which has its leading end passing through an aperture in nozzle 10 to the inside of nozzle 10, has an inlet 18 and an outlet 19 located inside nozzle 10. Although outlet 19, which is connected to a wide angle spray nozzle 20, is shown as being carried on the underside of diffuser plate 12, it can in fact be located in any place over the length of nozzle 10 also, if required the diffuser plates 12 and 13 maybe omitted. The microspheres and potassium carbonate solution required in the process are introduced through inlet 18 to pass down tube 17 and via outlet 19 to issue in atomised form from nozzle 20 into the ammonium nitrate spray passing from diffuser plate 12 to diffuser plate 13. Although the silica colloid required in the process can be added to the bulk nitrate solution before prilling, the preferred method is to make a solution of the silica colloid and to add this solution either on its own or in conjunction with the carbonate solution to the pnlling nozzle 10 via inlet 11. This late addition in the prilling process prevents gelling of the silica colloid and blockages in process equipment. Table 1 reflects some of the properties found with a product according to the invention. Microspheres Mass % Added Prill Density g/cm 3 Breaking Strength Kg Friability (%) Density After Loading g/cm 3 0 (Standard) 0,760 1,1 2,4 1,05 0,11 0,676 0,88 2,1 1,01 0,22 0,610 0.66 0,80 0,96 0,25 0,603 0,61 0,64 0.88 0,31 0,545 0.54 1,0 0,92 The microstructure of conventional PPAN and PPAN prepared according to the invention are respectively shown in the enclosed copies of two electron microscope photographs Nos 1 and 2, the encapsulated microspheres in the latter being marked 'X'. The enlargement of photograph No 1 is 2½ times that of photograph No 2. Both photographs show the presence of natural voids marked 'Y', which of course tend to disappear on pneumatic loading of the product. The product according to the invention can be used in conventional manner to manufacture a typical 94 :6 ANFO - type explosive composition. Table 2 reflects the detonation results obtained with such a composition. It will be noted from Table 2 that the velocity of detonation of the composition according to the invention is generally lower than that of the standard. Also, the critical diameter of the product is smaller while the sensitivity to initiation is higher than that of the standard, under charging conditions used during evaluation. Sensitivity to mechanical handling was tested by methods in accordance with those recommended by the United Nations, relating to the Transportation of Dangerous Goods, and the product according to the invention was found to be comparable with the standard. Applicant has found that a safe and highly efficient low cost ANFO explosive is provided according to the invention which shows enhanced sensitivity to initiation and consistent detonation velocities over a wide range of blast hole diameters and pneumatic loading pressures. Also, in the case of heavy ANFO. the low effective particle density of the product according to the invention offers an advantage over the known systems. This is so because the performance of heavy ANFO systems is vitally dependent upon the sensitivity of the blend when closed to static and dynamic pressures. It will be appreciated that there are no doubt many variations in detail possible with a novel product and method according to the invention without departing from scope of the appended claims.

Claims: A crystalline porous prilled ammonium nitrate product which includes hollow polymer microspheres being polymer balloons which had expanded during the prilling process to a size between 2 and 150 micro meter, incorporated in the crystalline structure. The porous prilled product of claim 1 wherein the microspheres have the following physical properties in the prilled product: Density range: 0,015 - 0,39 gram per cm 3 Temperature stability: Stable at process temperatures of 130-170°C for a sufficient time to effect prilling during the prilling process Breaking strength: Able to withstand at least 100 kg/cm 2 force or, able, to regain its shape after impact deformation. The porous prilled product of any one of the preceding claims which includes limestone. The porous prilled product of claim 3 wherein the concentration of the lime stone is between 0,1 and 30% mass per mass. The porous prilled product of any one of the preceding claims of which the porosity had been enchanced by the inclusion of gas in the product during the prilling of the product. The porous prilled product of claim 5 wherein the gas is developed in situ in the product via a suitable chemical reaction. The porous prilled product of claims 5 or 6 wherein the gas comprises carbon dioxide which is formed through the decomposition of a suitable carbonate in acid medium. The porous prilled product of claim 7 wherein the carbonate comprises any suitable water soluble inorganic salt of carbonic acid, such as, potassium and/or sodium carbonate or, alternatively, a less soluble salt. The porous prilled product of claim 7 wherein the carbonate comprises potassium carbonate which is present in a concentration of between 0,01 and 1,00% mass/mass. The porous prilled product of any one of the preceding claims which includes colloidal silica which upon drying during the prilling of the product migrates with the water in the prills to the surface to harden the latter and so improve the mechanical strength of the product. The porous prilled product of claim 10 wherein the colloidal silica is present in a concentration of between 0,1 and 10% mass/mass. A blasting composition including a porous prilled ammonium nitrate according to any one of claim 1 - 11. The blasting composition of claim 12 comprising ANFO or Heavy ANFO.

Inventors: BALS EDWIN, BREEDT JACOBUS, GOOSEN ADRIAAN JOHANNES, SPITERI WILLIAM LUCIANO, BALS, EDWIN, BREEDT, JACOBUS, GOOSEN, ADRIAAN JOHANNES, SPITERI, WILLIAM LUCIANO
Applicants: SASOL CHEMICAL IND LTD, SASOL CHEMICAL INDUSTRIES LIMITED
Application year: 1994
Main classification: B01J2
IPC codes: C06B31, C06B45, B01J13, C01C1